STaSIS Speed World Challenge Touring Car – Mid-Ohio
Faster than BMW, Acura, Mazda, Lexus or Honda, STaSIS proved at Mid-Ohio that an Audi A4 can outrun the best of the best in the most competitive road race series in the United States.
With a lap time of 1:40.843 Paul Lambert edged out Will Turner by 0.1 second for the pole position in the fourth race of the Speed World Challenge Touring Car Championship. With advice from Michael Galati coming moments before the session began Paul turned up the wick early to take advantage of degrading track conditions and set his fast lap on the second pass of the timing gate.
It was an all or nothing strategy as Crew Chief Larry Childs had set the car up with fresh tires which tend to go slick after a few laps. He added an aggressive differential setting to put down power out of the tight turns and minimal downforce to optimize top speed. It took Paul one lap to tame the high speed oversteer through turn one resultant from the lack of downforce, but he pummeled the tight parts of the track and gained 3 mph down the back straight.
The STaSIS Touring Car has made tremendous progress in the last four races, adding something new for each event. This event saw significant changes in the differential tuning that will be outlined later in this article. Other improvements to the suspension, brake system, and engine will be reviewed as we cover each race in the series.
The quick race summary puts Paul in the lead for the first eight laps of the race as he and Will Turner pulled away from the pack by five or ten car lengths. A flat tire eventually denied the potential win, but the strength of the first laps proved that the STaSIS Audi A4 is real contender.
Power Steering Issues
Thursday testing saw the STaSIS Audi running in the top 10 but with considerable discomfort from power steering issues. The first session out the power steering would come and go at differing positions on the track resulting in situations like strong power assist on the straights and none in the turns. The mid-turn transition from assist to non-assist is quite a challenge at competitive speeds and the team smartly focused on this issue first.
The power steering pump had been changed to a lightweight / low-drag racing unit and this was immediately under suspicion. Cavitation caused by fluid starvation in the turns was suspected and the system was bled and re-filled to a higher level.
In the second session out the power steering was over-boosted in the turns and would cut out on the straights. Again the abrupt transition from no assist to super-assist on corner entry was not tolerable and the team went back to look for more answers. A complete tear down of the power steering system revealed a faulty steering rack. Small metal particulates were jamming the torsion arm, which is the piece that controls the hydraulic application. The rack was changed and the car completed its third session with excellent steering response and all fears of the lightweight pump put aside.
Handling and Driveline Tuning
The next issue to be addressed was handling. The car had good mid-corner balance but suffered from a power on oversteer with early power application. Since the strength of the Audi quattro is its ability to put down power earlier than the competition this became a focus. Sway bar changes were made for the next session improving the early power oversteer but adding a corner exit understeer. Similar to a rear drive vehicle, early power application would cause rear wheel spin-up inducing oversteer, followed by typical front wheel drive understeer as the car would straighten out and spin up the front tires.
Data acquisition told the story clearly. The car had initially suffered from a power on understeer condition, which could not be balanced with the suspension. This is shown in the Pi traces as front inside wheel spin from throttle application to corner exit.
The center torsen was then changed from an approximate 3:1 bias ratio to 5:1. This enabled the car to put more power to the end that had more traction. The car was now overpowering both inside tires, front and rear. The Pi trace showed the car trying to spin up the rear tire and the torsen moving power to the front tires. The torsen was doing an excellent job of equalizing the use of front and rear grip in a few fractions of a second. Unfortunately the A4 was still traction limited.
At this point the car was limited by the combined traction of the front and rear inside tires. The rear differential was then changed from a 35lb preload to a 75lb preload. This enabled more power to be applied to the loaded side of the rear suspension. With this increased grip the torsen automatically proportions more power to the rear, minimizing wheel spin to both front and rear. This is the Pi trace we wanted.
The car was finally neutral from corner entry to exit and power could be applied 2-3 car lengths before either a rear wheel or front wheel drive vehicle. This setup along with a trim of the rear wing set the pole at 1:40.843.
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